(1) Field of the Invention
The present invention relates generally to the control of pests that cause damage to corn plants by their feeding activities, and more particularly to the control of such corn plant pests by the combination of a corn seed having a transgenic event and the treatment of such seed with a thiamethoxam pesticide prior to planting the seed.
(2) Description of the Related Art
Insects and related arthropods annually destroy an estimated 15% of agricultural crops in the United States and even more than that in developing countries. In addition, competing weeds and parasitic and saprophytic plants account for even more potential yield losses.
Some of this damage occurs in the soil when plant pathogens, insects and other such soil borne pests attack the seed after planting. In the production of corn, much of the rest of the damage is caused by rootworms—insect pests that feed upon or otherwise damage the plant roots; and by cutworms, European corn borers, and other pests that feed upon or damage the above ground parts of the plant. General descriptions of the type and mechanisms of attack of pests on agricultural crops are provided by, for example, Metcalf, in Destructive and Useful Insects, (1962); and Agrios, in Plant Pathology, 3rd Ed., Academic Press (1988).
Corn is the most important grain crop in the Midwestern United States. Among the most serious insect pests of corn in this region are the larval forms of three species of Diabrotica beetles. These include the Western corn rootworm, Diabrotica vergifera vergifera LeConte, the Northern corn rootworm, Diabrotica berberi Smith and Diabrotica berberi Lawrence, and the Southern corn rootworm, Diabrotica undecimpunctata howardi Barber. In fact, more insecticide is used for the control of corn rootworm than for any other pest of corn, and the total acreage treated is greater than for any other pest in the United States.
Corn rootworms (CRW) overwinter in the egg stage in fields where corn was grown the previous season. The eggs hatch from late May through June. If a corn crop is not followed by another corn crop in the subsequent year, the larvae will die. Accordingly, the impact of corn rootworm is felt most directly in areas where corn is systematically followed by corn, as is typical in many areas of the Midwestern United States.
After hatching, the larvae pass through three larval stages or instars, during which they feed on the corn root system. About three weeks is required for completion of the larval stage. Damage to the corn root system caused by the feeding of larvae is the major cause of harvest losses in corn due to corn rootworm. Corn plants that fall over and lodge in the field after weakening or destruction of a major part of the root system are the cause of a major portion of this loss, since this lodged corn cannot be harvested by conventional mechanized machinery and is left in the field.
Following completion of larval development, the larvae transform into immobile pupae, and thence into the adult beetles that emerge from the soil throughout the summer, with the period of emergence depending upon the growing location. After emergence, the adult beetles feed for about two weeks before the females start laying eggs. Initially, the adults feed predominantly in the same field from which they emerged, but later will migrate to other fields. Peak adult activity normally occurs in the U.S. Corn Belt during late July or early August in fields planted to continuous corn, but activity may peak later in first year or late maturing cornfields. Rootworm beetles begin depositing eggs in cornfields approximately two weeks after they emerge. (For more information, see, e.g., Corn Rootworms, Field Crops Pest Management Circular #16, Ohio Pest Management & Survey Program, The Ohio State University, Extension Division, Columbus, Ohio; available online at www.ag.ohio-state.edu/˜ohioline/icm-fact/fc-16.html, Sep. 13, 2000; and McGahen et al., Corn Insect Control: Corn Rootworm, PENpages number 08801502, Factsheet available from Pennsylvania State University, State College, PA, 1989).
In present conventional agricultural practice, in cases where corn follows corn, it is normal for an insecticide to be applied to protect the corn root system from severe feeding by rootworm larvae. Conventional practice is to treat for the adult beetles or to treat for the larvae. Examples of conventional treatment formulations for adult beetles include the application of carbaryl insecticides (e.g., SEVIN® 80S at 1.0-2.0 lbs active/acre); fenvalerate or esfenvalerate (e.g., PYDRIN® 2.4EC at 0.1 to 0.2 lbs active/acre, or ASANA® 0.66EC at 0.03 to 0.05 lbs active/acre); malathion (57% E at 0.9 lbs active/acre); permethrin (e.g., AMBUSH® 2.0EC at 0.1 to 0.2 lbs active/acre, or POUNCE® 3.2EC at 0.1 to 0.2 lbs active ingredient/acre); or PENNCAP-M® at 0.25-0.5 lbs active/acre.
To treat for CRW larvae, conventional practice is to apply a soil insecticide either at or after planting, but preferably as close to egg hatching as possible. Conventional treatments include carbofuran insecticides (e.g., FURADAN® 15G at 8 oz/1000 ft of row); chloropyrifos (e.g., LORSBAN® 15G at 8 oz/1000 ft of row); fonophos (e.g., DYFONATE® 20G at 4.5 to 6.0 oz/1000 ft of row); phorate (e.g., THIMET® 20G at 6 oz/1000 ft of row); terbufos (e.g., COUNTER® 15G at 8 oz/1000 ft of row); or tefluthrin (e.g., FORCE® 3G at 4 to 5 oz/1 000 ft of row).
Many of the chemical pesticides listed above are known to be harmful to humans and to animals in general. The environmental harm that these pesticides cause is often exacerbated due to the practice of applying the pesticides by foliar spraying or direct application to the surface of the soil. Wind-drift, leaching, and runoff can cause the migration of a large fraction of the pesticide out of the desired zone of activity and into surface waters and direct contact with birds, animals and humans.
Because of concern about the impact of chemical pesticides on public health and the health of the environment, significant efforts have been made to find ways to reduce the amount of chemical pesticides that are used. Recently, much of this effort has focused on the development of transgenic crops that are engineered to express insect toxicants derived from microorganisms. For example, U.S. Pat. No. 5,877,012 to Estruch et al. discloses the cloning and expression of proteins from such organisms as Bacillus, Pseudomonas, Clavibacter and Rhizobium into plants to obtain transgenic plants with resistance to such pests as black cutworms, armyworms, several borers and other insect pests. Publication WO/EP97/07089 by Privalle et al. teaches the transformation of monocotyledons, such as corn, with a recombinant DNA sequence encoding peroxidase for the protection of the plant from feeding by corn borers, earworms and cutworms. Jansens et al., in Crop Sci., 37(5):1616-1624 (1997), reported the production of transgenic corn containing a gene encoding a crystalline protein from Bacillus thuringiensis (Bt) that controlled both generations of the European corn borer. U. S. Pat. Nos. 5,625,136 and 5,859,336 to Koziel et al. reported that the transformation of corn with a gene from B. thuringiensis that encoded for delta-endotoxins provided the transgenic corn with improved resistance to European corn borer. A comprehensive report of field trials of transgenic corn that expresses an insecticidal protein from B. thuringiensis has been provided by Armstrong et al., in Crop Science, 35(2):550-557 (1995).
It was known that wild-type Bt δ-endotoxins had low activity against coleopteran insects, and Kreig et al., in 1983, reported the first isolation of a coleopteran-toxic B. thuringiensis strain. (See U.S. Pat. No. 4,766,203). U.S. Pat. Nos. 4,797,279 and 4,910,016, also disclosed wild-type and hybrid B. thuringiensis strains that produced proteins having some coleopteran activity. More recently, however, more precise genetic engineering methods have shown promise in developing modified B. thuringiensis proteins that have significantly higher levels of corn rootworm activity than those produced by wild-type parents. (See, e.g., WO 99/31248 to Ecogen, Inc. and Monsanto Company).
However, it is not known at present whether any transgenic event alone will be sufficiently effective to protect corn from damage by corn rootworm in heavily infested fields that are dedicated to serial corn. In fact, the total control of corn rootworm damage by any one transgenic event may not be desirable in the long term, because of the potential for the development of resistant strains of the target pest.
Another alternative to the conventional forms of pesticide application is the treatment of plant seeds with pesticides. The use of fungicides to protect seeds from attack after planting, and the use of low levels of insecticides for the protection of, for example, corn seed from wireworm, has been used for some time. Seed treatment with pesticides has the advantages providing for the protection of the seeds, while minimizing the amount of pesticide that was required and limiting the amount of contact with the pesticide and the number of different field applications that were necessary.
Other examples of the control of pests by applying insecticides directly to plant seed are provided in, for example, U.S. Pat. No. 5,696,144, which discloses that the European corn borer caused less feeding damage to corn plants grown from seed treated with a 1-arylpyrazole compound at a rate of 500 g per quintal of seed than control plants grown from untreated seed. In addition, U.S. Pat. No. 5,876,739 to Turnblad et al. (and its parent, U. S. Pat. No. 5,849,320) disclose a method for controlling soil-borne insects which involves treating seeds with a coating containing one or more polymeric binders and an insecticide. This reference provides a list of insecticides that it identifies as candidates for use in this coating and also names a number of potential target insects. However, while the U.S. Pat. No. 5,876,739 patent states that treating corn seed with a coating containing a particular insecticide protects corn roots from damage by the corn rootworm, it does not indicate or otherwise suggest that such treatment could be used with seed having a transgenic event.
The treatment of seed having a transgenic event with nitroimino- or nitroguanidino-compound pesticides has been mentioned (See, e.g., WO 99/35913), however, no guidance has been found as to the potential utility or efficacy of such treatments, or the details of how such treatments might be effected—such as the amounts of active ingredient that would be necessary per unit amount of seed—and no examples that would give reason to believe that the proposed treatments would actually provide suitable protection.
Therefore, although recent developments in genetic engineering of plants have improved the ability to protect plants from pests without using chemical pesticides, and while such techniques as the treatment of seeds with pesticides have reducing the harmful effects of pesticides on the environment, numerous problems remain that limit the successful application of these methods under actual field conditions. Accordingly, it would be useful to provide an improved method for the protection of plants, especially corn plants, from feeding damage by pests. It would be particularly useful if such method would reduce the required application rate of conventional chemical pesticides, and also if it would limit the number of separate field operations that were required for crop planting and cultivation.